Power limits per circuit?

[mkrohn]

Are there any good threads around here about power? Right now I'm having to use multiple locations for my rigs to avoid possibly popping the circuit. If I use a kill a watt device and measure how much each rig is using at the outlet how many watts can I safely stack on each circuit?

This week at some point I will be building shelving racks and mounting fans flush against the racks both pushing and pulling air over my precious video cards.

Where I am building is in the same room as the circuit breaker and in close proximity to the furnace. Some might call it a closet but I grew up with a bedroom that size.... I will be putting in dedicated circuits straight to my shelving units and I'm trying to figure out how many I need to do. I planning on going all out and mounting a monitor on the rack and later on adding a KVM but its just not in the budget right this second.

I will be running the basement fan 24/7 to send the heat upstairs where it is needed. I've already closed and even taped off the OUTWARD flowing vents that are in the basement to stop the furnace from wasting any heat down there.

 

[Core32]

You need to provide more information.
Electrical panels have an overall limit, just as each breaker within the panel will have an individual limit. And each panel will have a physical limit on the number of breakers it can actually fit.
Within my panels I have a variety of 10, 15 and 20 amp breakers.
What does your panel presently look like and what is it's maximum rating?
Disclaimer!!!!
Let me warn you not to run the circuitry yourself from the panel if you are not an experienced electrician or even add wire to existing circuitry.
This is a house fire waiting to happen if you do it wrong and it is easy to do it wrong with the amounts of current you are working with.

 

[_r2h]

I usually don't just straight to the "hire a professional" if you need work, but this is one of the few instances where I'd recommend that. There are a lot of variables, no sense in burning your house down when it can be prevented.

Largest I'd use is a 30amp breaker, the appropirate 30amp receptacle (think washing machine type), with a large guage wire (tons of charts out there for load, wire material, etc). That'd allow you to run 2 x 1600watt number crunching machines, with a very small headroom margin.

 

[bigfootape]

There are charts out there outlining wire size for a given load. The only charts that matter are referenced by your electrical code.

If you're using a PDU, you can size your supply circuit for that using the appropriate wire and breaker deratings from your local electrical code.

 

[mkrohn]

My father in law is a licensed contractor so he knows all of the codes but hey he's my father in law, double checking on stuff is always smart.

 

[Dogs]

If I use a kill a watt device and measure how much each rig is using at the outlet how many watts can I safely stack on each circuit?

If you want an approximate number, basic physics should do as you need. P=IV. You know whether the the circuit is 120 or 220 volts, and you should know how many amps the circuit is limited to, so you know the maximum theoretical power that circuit can handle. If there's nothing else on that circuit, you can get somewhere close to it, but you should leave some headroom. If you're doing 30 amps at 120 volts, that gives you 3600 watts to play with. If together your machines use about that much, I wouldn't put them on the same circuit as each other. But if theres a few hundred watts leftover after it's all said and done, it should be safe.

If you're not using an existing circuit, you're probably going to want an electrician or someone who knows what they're doing to put in the ideal circuit. If you get it wrong, such as using too thin of wire, (Electrical resistance is inversely proportional to wire cross sectional area, so thinner wire means higher resistance, and higher resistance means more electricity gets converted into heat. At 30 amps, we're talking an easy house fire), your insurance company and your local fire department are not going to like you.

If you go trying to measure the outlet with a meter (other than something extremely consumer friendly), make sure you know what you're doing. Ideally, you'd be best using a meter that plugs into the outlet, and has an outlet for the device to plug into, since using probes on an electrical outlet has steep concequences for error.

 

[bman212121]

A kill a watt would be the best place to start. Definitely get one of those before your father in law even comes over and get an idea what kind of power the boxes are drawing. Write down all of the boxes wattage and amperage onto a piece of paper.

As stated before without knowing what kind of load you have it would be impossible to give any type of recommendation.

For instance, one pc with only a cpu and graphics card at idle could be as low as 50W. It might be 120W with just the cpu running, and could jump right up to 320W with a single high end GPU. Throw two more gpus in there and now it's 720W or more.

That said at the most basic level to get an idea of what you might be able to run, you just need to know a simple formula.

Voltage * Amperage = Wattage

Standard voltage is considered to be around 120 volts but it can fluctuate several volts in either direction and still be considered in spec by the power company.

A standard household circuit is generally 15amps, with areas like the kitchen using 20amps. The thing with most circuits in a house is that they tend to randomly jump all over the place and you might not know what else is feeding off of it. So it would NOT be a good idea to assume you can use the full power of that circuit if it is not dedicated.

So assuming you have a dedicated 15 amp circuit put in (Dedicated as in the only thing plugged into it is your computers), and your house is running 120V, you would have this:

120 * 15 = 1800

So a dedicated circuit could provide up to 1800 continuous watts in a typical scenario. Now if you are planning 24 / 7 usage I wouldn't be pushing it right to the limit for a few reasons. As I said before voltage can fluctuate. If the voltage dips down to say 115V were only working with ~1725W.

115 * 15 = 1725

And voltage also tends to sag a little when you load a circuit heavy so even under normal conditions it might be 1 or 2V less under load.

So if we stay on the safe side we can put a cap of around 1600W for a dedicated 15 amp circuit.

Going back to the scenario before, you could likely run 4 computers with a single high end GPU and stay within the limit. If you're talking TRI SLI then it's a whole different scenario and you might only be able to run 2 boxes on one circuit. It's essential to know exactly what you are hooking up to the circuit in order to decide how the load needs to be split up.

Hopefully this info should help give anyone who was thinking the same thing a few ideas of what they should be doing. For the OP once you get those numbers your father in law can easily take that info and help you determine your current needs and your future needs and plan out the outlets accordingly.

For anyone thinking about 30 amp circuits for home use I definitely would NOT recommend it! The issue is that a 30 amp circuit will protect the wiring and outlets that are plugged into it because they should be sized correctly. (I don't think you can actually get a 30 amp outlet that would accept the typical 15 amp style plug your devices use anyway) So if you plug in a light duty extension cord like a common 16AWG it's only rated to 13 amps. Nothing is going to stop you from pulling 30 amps through that cord since the breaker was designed for a much larger 10awg wire. The cord is likely going to go up in flames since it's over 2x the draw of what it was rated for. You are much better off just using 2 separate 15 amp circuits as you will have the same amount of total power output, but it will provide a bit better protection from people doing silly things because the breaker will trip with a lot less current flowing through it.

 

[mkrohn]

Thank you very much for all of that info. I now have to decypher the names of the various rooms and do some outlet checking with like a clock or something but I'll wait till kids are back from school to have a helper watch the clock and come tell me.

The room I would LIKE to do everything in is on a 15A but I was thinking about building within about 10ft of the circuit box. I have 4 spots available at the box.

There's a room somewhere in this house with a pair of 30A going to it. I'm guessing it is probably my sons room because it was the previous homeowners "office". There is also a 20A going to the other main room in the basement which is full of kids toys.

The majority of my rigs are TWO video card setups in the 500-600w range mostly of actual usage at the outlet. I have ONE that pulls around 900w from the wall.

Thank you guys, I'm going to do quite a bit more reading and I have some more systems I'm bringing online tuesday/wednesday so now I have a better idea of which rooms I can even use until I build the rack and actually wire it up.

Anything crazy have to be done to upgrade that 15w to say a 30 or is that mainly limited by what wire is in the wall and don't even think about touching?

Also about the random stuff using various circuits. My father in law actually build this house in the early 90's with the guy who was the owner. The previous owner was INSANELY anal about doing this "right" Everything in the circuit box is just in a certain area and is very well labeled. Of course the names of rooms are what I have to decypher but I can almost certainly count on the circuits to be dedicated if I decide to build based on existing wiring.

 

[Core32]

....and do some outlet checking with like a clock or something but I'll wait till kids are back from school to have a helper watch the clock and come tell me.....

Use a very loud AC powered radio

 

[mkrohn]

Also I'm not going to use ANY extension cords or anything silly like that. So in my situation I'm going to have a rack with 6 dual GPU rigs lets just say each using 600w In that situation I would actually need to use THREE 15A lines? since the 3600w of 6 of them is at the limit. What I would do to counter that is have a lower power consumption machine like say 2 6950's

I just used a calculator and it looks like
dual 7970's is just under 500w
dual 7950's is just under 400w
dual 6950's is 400w
dual R9 280x is listed about 550w

I know at least the R9's are stock overclocked so will likely be quite a bit higher. most of my cards actually I think are overclocked.

Sounds like I will be writing a pretty detailed report on all of this. I haven't posted anything on iamnotageek.com in forever so I guess it's just about time to. I have to write down all of this information anyway so I might as well share it.

 

[J Macker]

OP,

Also remember that psus are more efficient when input is 240v.
Efficiency is everything for a mining operation. Find the peak efficiency for the psu (in percent load) and try to get the right power supply for the gpus.

If you're using 4x 7970's or 280x cards, then a 1300w psu will put you right around 90% load.
From the wall it will pull about 10% more, so if you use 90% of your psu, it will pull about it's rating at the wall.

Put in a double 20a to make a 240v and you will have 4800w
Divide that by however many watts per gpu and add roughly 75w for the cpu/mobo and you're set.
A typical 4x 7950 rig will pull about 1000w at the wall. 4x Radeon 280x or 7970 will pull 1200-1300w.

 

[Dogs]

Anything crazy have to be done to upgrade that 15w to say a 30 or is that mainly limited by what wire is in the wall and don't even think about touching?

The wires in the wall will be a limiting factor, yes. If you don't want to get new wires put in, you're better off just doing multiple 15 amp circuits instead of one big 30 amp circuit. That will also have the added benefit that if for some reason a breaker/fuse does trip, it's only taking down 1 or 2 boxes instead of all of them.

Also remember that psus are more efficient when input is 240v.
Efficiency is everything for a mining operation.

The cost of safely moving to 240v for those machines (and by safely, I mean getting an electrician to do it) would be quite high. Even with the added efficiency, it would take a while for that to pay for itself in savings.

 

[Dogs]

There's a room somewhere in this house with a pair of 30A going to it. I'm guessing it is probably my sons room because it was the previous homeowners "office". There is also a 20A going to the other main room in the basement which is full of kids toys.

Just caught this. No, there is no room in your hourse that has a pair of 30A. That's a double pole breaker that trips at 30A, so something in your house has 240V with a 30A fuse. This is probably your dryer or your furnace or something, and you'll want to leave it alone.

 

[mkrohn]

Just caught this. No, there is no room in your hourse that has a pair of 30A. That's a double pole breaker that trips at 30A, so something in your house has 240V with a 30A fuse. This is probably your dryer or your furnace or something, and you'll want to leave it alone.

not sure what that one is. Do well pumps tend to be 240v?

My sons room did actually have both a 15a and a 20a going to it. That room used to be the guys office and he has a 15A going to where his computer was and another 20A for that room itself. There is a corner area that has outlets for both circuits. He usually sleeps with a fan on... SOOO should I give him additional fan noise? I think I'd surround it with like a baby coral type thing if I put it in there. This option is of course the least work and requires no electrical fiddling.

I also have a 20A circuit in what I use as an office. A ceiling light/fan and my computer are what get used here. I think if I had a couple in here I'd just plug the vacum into the other room.

 

[J Macker]

stop using the vacuum.

 

[_r2h]

Just caught this. No, there is no room in your hourse that has a pair of 30A. That's a double pole breaker that trips at 30A, so something in your house has 240V with a 30A fuse. This is probably your dryer or your furnace or something, and you'll want to leave it alone.

You can have single pole 30a, washers use them.

 

[Dogs]

You can have single pole 30a, washers use them.

Please point out to me precisely where I said that single pole 30 A breakers don't exist, because I don't recall ever saying that anywhere in my entire life.

 

[plext0r]

Something to remember is that your circuit needs to be sized for 125% of the continuous load. In other words, your load should only pull 80% of the circuit breaker size. This is part of the US Electric Code and is designed for safety. Yes, you can pull close to 20A from a 20A circuit without frying anything, but it's not best. If you put standard 20A circuits in your closet, you should only plan to continuously pull 16A from each one. Likewise you should only pull 12A from a 15A circuit.

Here's one article I found informative on the subject:
http://www.electrical-contractor.net/forums/ubbthreads.php/topics/128677/80_breaker_rating.html

 

[bman212121]

Thanks for that link brilong!

Very interesting read as I can't say as I've seen anything talking about 24/7 requirements. I know first hand that it's possible to run 28A for an extended amount of time through a 20A breaker, ie kitchen appliances (It took someone in the house 4 appliances and 37A to actually trip it ) I was thinking breakers were actually spec'ed for about 125% so if it's a 20A breaker would take a continuous 25A to get it to trip. Seems that's not the case at all and it's the other way around.

So on a 15A circuit you actually want to be below 12A or 1440W.

 

[Bill1024]

I would run 2. 20 amp outlets.
If you run a 30 amp outlet make sure you get a 30 amp receptacle.
Regular outlets are rated 15 amps and 20, I have seen 15A on a 20 circuit,
they do use them on 20 amp circuits in many locals, check local code.
If there are no local codes go by the National electric code.
If you haven't run wire before it is best to have someone else do it, or help you.
Just a few tips.
Black to the breaker, white to the common bar and green to the grounded bar.
You will see how the other breakers are done. Main panel the white and green are bonded.
On the plug end.
Black goes to the brass screw, white to the silver screw and green to the green screw.
In the box leave an extra 6 inches so the receptacle can be worked on outside the box.
Wires have to be twisted together, a wire nut is not a connector, it is an insulator!!!!!
For Gods sake work with the power OFF. Use a meter to make sure it is off.
Wire size is as follows
#14 wire for 15 amps
#12 wire for 20 amps.
#10 wire for 30 amp

 

[Hagrid]

I would run 2. 20 amp outlets.
If you run a 30 amp outlet make sure you get a 30 amp receptacle.
Regular outlets are rated 15 amps and 20, I have seen 15A on a 20 circuit,
they do use them on 20 amp circuits in many locals, check local code.
If there are no local codes go by the National electric code.
If you haven't run wire before it is best to have someone else do it, or help you.
Just a few tips.
Black to the breaker, white to the common bar and green to the grounded bar.
You will see how the other breakers are done. Main panel the white and green are bonded.
On the plug end.
Black goes to the brass screw, white to the silver screw and green to the green screw.
In the box leave an extra 6 inches so the receptacle can be worked on outside the box.
Wires have to be twisted together, a wire nut is not a connector, it is an insulator!!!!!
For Gods sake work with the power OFF. Use a meter to make sure it is off.
Wire size is as follows
#14 wire for 15 amps
#12 wire for 20 amps.
#10 wire for 30 amp

120v isn't too bad for being shocked. 240 is a good waker upper.

I would run 20 or 30's, that way you have plenty of headroom.
If you are doing it, better to do it only once.

 

[mkrohn]

Thanks for that link brilong!

Very interesting read as I can't say as I've seen anything talking about 24/7 requirements. I know first hand that it's possible to run 28A for an extended amount of time through a 20A breaker, ie kitchen appliances (It took someone in the house 4 appliances and 37A to actually trip it ) I was thinking breakers were actually spec'ed for about 125% so if it's a 20A breaker would take a continuous 25A to get it to trip. Seems that's not the case at all and it's the other way around.

So on a 15A circuit you actually want to be below 12A or 1440W.

yeah I started hooking up machines and have more going in the test area than I thought it would let me. I am about to move a couple of the machines to other locations. I still haven't built the rack. My father in law ordered a bunch of parts and lumber for for me and we'll do it the day things come in. Until then I'm trying to do everything I can to keep kids away from the machines that are basically on some shelving just sitting on bench areas

 

[mkrohn]

Finally popped that circuit. When I posted earlier I only added a single 6950 and right after it fired up my whole basement went dark. Apparently the lights down there are on that circuit I was using.

I'm scrambling to move stuff around and get back up but 4 6950's, 2 7970's and 4 280x's is too much for a 15A

 

[plext0r]

Finally popped that circuit. When I posted earlier I only added a single 6950 and right after it fired up my whole basement went dark. Apparently the lights down there are on that circuit I was using.

Yeah, you definitely want your rigs on a separate circuit from any lighting. You might also consider putting your ISP gear, home router, etc. on a APC UPS so they can weather any power outages or breaker trips.

 

[Hagrid]

Yeah, you definitely want your rigs on a separate circuit from any lighting. You might also consider putting your ISP gear, home router, etc. on a APC UPS so they can weather any power outages or breaker trips.

That is how I set mine up and works well.

That is a lot of video cards and might pull a few watts......

 

[bman212121]

Finally popped that circuit. When I posted earlier I only added a single 6950 and right after it fired up my whole basement went dark. Apparently the lights down there are on that circuit I was using.

I'm scrambling to move stuff around and get back up but 4 6950's, 2 7970's and 4 280x's is too much for a 15A

LOL, I think you need to go back and reread the last two pages of us talking about exactly what you shouldn't have just done!

Those 280x cards are power hungry all by themselves.

http://www.techpowerup.com/reviews/Gigabyte/R9_280X_OC/24.html

Showing ~216W a piece as measured from the DC side. Factor in a 90% efficiency on the PSU and it's more like 240W per card from the wall. So just the 4 cards is 960W!

2 7970 are about the same so that's another 480W putting you at your 1440W 24/7 load for a dedicated circuit and not including the rest of the system.

So you should put those 4 6950's onto another circuit and you might not blow it again.

 

[mkrohn]

LOL, I think you need to go back and reread the last two pages of us talking about exactly what you shouldn't have just done!

Those 280x cards are power hungry all by themselves.

http://www.techpowerup.com/reviews/Gigabyte/R9_280X_OC/24.html

Showing ~216W a piece as measured from the DC side. Factor in a 90% efficiency on the PSU and it's more like 240W per card from the wall. So just the 4 cards is 960W!

2 7970 are about the same so that's another 480W putting you at your 1440W 24/7 load for a dedicated circuit and not including the rest of the system.

So you should put those 4 6950's onto another circuit and you might not blow it again.

I moved the 2 280x's and the 2 6950's into the next room which is a 20A circuit. One of the 7970's is now upstairs in my desktop pulling 700k hash while the desktop is usable for a web browser at least. The biggest of the rigs is now just 1 7970 and 2 280x's. In that room with is is a big light that takes 91w but lights things up so nicely but a curly q lightbulb is the other option for light in there. i have another 6950 rig installing windows on that testbench area but I now have systems running with fans pointed on them throughout the house. I have to get a hold of that father in law cause this has to get done.

I had a final exam on tuesday and had a statistics test today along with preparing for the other finals coming next week.